So what exactly is circumstantial evidence? It's basically evidence that requires an inference to prove something.
The most common example is rain. Say you want proof that it was raining this morning. Direct evidence would be if someone came in and said, "I saw it raining this morning." Circumstantial evidence would be if the person said they heard distant pitter-patter and later walked outside and saw the ground was wet, and the air smelled fresh and felt moist.
Would you be any less inclined to believe it had rained under the second scenario? Probably not. Of course, the reliability of any evidence, whether direct or circumstantial, is only as good as its source.
For more in-depth information, check out my article on direct versus circumstantial evidence.